1. Field of the Invention
The present invention is generally directed to thin-film optical filters.
More specifically, it is directed to a method for realizing a filter edge of an optical thin-film filter at a desired spectral position, further to an optical thin-film which defines a desired spectral position of a filter edge between a spectral range wherein light is to be blocked and a spectral range wherein light is to be transmitted or reflected. It is further directed to an optical device which comprises an optical filter with at least one optical thin-film and finally to a sun-tanner apparatus which comprises an optical filter device between its ultraviolet source and its optical output where light emits for tanning purposes.
The problems which have been solved by the present invention were specifically recognized in the art of ultraviolet tanning technique. Therefore, such problems are here specifically discussed from the standpoint of that art. Nevertheless, the teaching of the present invention is generally valid on the above-mentioned general art of optical thin-film technology.
2. Description of the Prior Art
In plants for cosmetic purposes and/or therapy, it is known to use ultraviolet high-pressure sources. Such sources in fact emit beside of desired radiation in the UVA spectral range, radiation in the UVC and in the UVB spectral range, as well as in the visible spectral range and in the near infrared spectral range. These spectral ranges are defined as follows:
UVC: 100 nm .ltoreq..lambda..ltoreq.280 nm; PA0 UVB: 280 nm .ltoreq..lambda..ltoreq.315 nm; PA0 UVA: 315 nm .ltoreq..lambda..ltoreq.380 nm; PA0 Visible radiation (VIR): 380 nm .ltoreq..lambda..ltoreq.780 nm; PA0 near infrared radiation (NIR): 780 nm .ltoreq..lambda..ltoreq.1.4 .mu.m. PA0 0.25.ltoreq.y/(x+y).ltoreq.0.5, thereby preferably to PA0 0.25.ltoreq.y/(x+y).ltoreq.0.45 and even in a preferred mode to be approx. 0.35, thereby selecting the desired spectral position between UVA- and UVB-spectral range.
From the standpoint of human health UVC- and UVB-radiation is not desired and from the standpoint of comfort of persons exposed to that radiation a too strong radiation in the visible and in the near infrared spectral range is also not desired. Therefore, it is customary to perform optical filtering on the radiation emitted from such a source, thereby only transmitting at maximum transmission the desired UVA spectral range radiation.
It is known to use for the realization of such filters so-called ultraviolet glasses, as for instance Uvisol (trademark) M-UG2 of the firm "Deutsche Spezialglas AG". If such ultraviolet glasses (UV-glass) are used, there must be used a clear glass with an according absorption edge so as to accurately block the UVB-radiation which, as known, causes erythema and which, considered in the spectrum, directly follows the UVA-radiation range to be exploited.
The drawbacks of such UV-glasses are that they lead to a certain amount to solarization, i.e. to a shift of the transmission behaviour due to radiation influences and the relatively low transmission within UVA spectral range which is to be exploited. Solarization of the clear glass leads to a shift of its absorption edge toward longer wavelengths so that, during operation, the UVA-radiation part will additionally diminish in an uncontrolled manner. Due to the relatively low UVA-transmission, it is necessary to use UV burners with very high power, so for instance of 2 to 4 kW. This further leads to the necessity that huge efforts and expenditures must be done for appropriate cooling of the apparatus and for generating and applying the high electric power.
Since some time, multilayer interference filters are on the market, for instance produced by applicant of the present invention, which filters consist of alternating thin layers with high and low indices of refraction, so for instance made of Ta.sub.2 O.sub.5 and of SiO.sub.2, respectively. Applied as UVA-band-pass filters, they exhibit a significantly higher UVA-transmission than may be reached with UV-glass filtering technique.
Due to the use of thin metal oxide layers for such optical interference filters, no solarization occurs. A further significant advantage of such interference filters, compared with UV-glass filters is, that the layer material is substantially free of absorption so that a very high UVA-transmission is reached whereby the filtering edges toward the visible spectral range on the one hand and toward the UVB-spectral range on the other hand may be tailored very steep by exploiting interference phenomena. By varying the thickness of the layer, it becomes further possible to shift the filter edges to such spectral positions that the convolution of the transmission curve with the pigmentation curve becomes maximum which leads to optimal tanning and so that the convolution with the erythema curve becomes minimal which leads to the avoidance of sun-burn.
With respect to the definition of the said pigmentation curves and of said erythema curves, attention is drawn on DIN 5031, part 10 and on DIN 5050.
By the use of such interference filters and additional use of coated UVA-reflectors, the required electrical power of some UV-radiation apparatus was significantly reduced, also due to high UVA-transmission.
In the art of such interference filters, there is provided on both sides of a glass substrate a multi thin-layer system. One of them provides for blocking UVB- and UVC-radiation, the other of them blocks visible radiation and radiation in the near infrared spectral range. Primarily based on the both-sided thin-layer coating, the drawback resulting from such systems is that a relatively large number of thin layers, approximately 70, is required which leads to comparatively high manufacturing costs.
In the following description and claims, we understand under the term "thin-film" a film or layer which is produced or deposited on a substrate by a vacuum-coating process, as for instance by a physical vapor deposition process (PVD) or a chemical vapor deposition process (CVD) or a plasma enhanced chemical vapor deposition process (PECVD) or the like. Additionally, such a layer may also be produced by a CVD process at higher than atmospheric pressures.
Considered from the standpoint of general optical filter art, it is known from the FR-A-2 362 412 which accords to the U.S. Pat. No. 4,158,133 and further from the FR-A-2 626 981 a filter technique which makes use of so-called "absorption filters" and of interference filters. The so-called "absorption filters" are in fact described and would also be understood by the man skilled in that art as coloured glass. Thus, these references make use of absorption filters which are not based on thin-film and which do not contribute to the filter action of the thin-film interference filters.
Further attention is drawn to the EP-A-0 410 160 according to the U.S. Pat. No. 5,138,485.